The purpose of this study was to test a conceptual model based on theoretical and empirically supported relationships related to the influences of weight perceptions, weight concerns, desires to change weight, friends, age and location in relation to physical activity (PA) and smoking in adolescents. A total of 1242 males and 1446 females (mean age = 15.6 [plus or minus] 1.3) were recruited from rural and urban Canadian schools. Study respondents provided self-reports of PA, "smoking", "perceived body weight", "desire to change weight", "concern about weight gain" and "friends' smoking and PA behaviors". Results revealed an acceptable fitting model [chi][squared] (40) = 155.63, P less than 0.05, root mean square error of approximation = 0.047 and comparative fit index = 0.98. Large effect sizes for both genders were observed between friends' and adolescents' smoking behavior, and between perceived body weight and desire to change weight. Further, significant differences were identified between the male and female models [[chi][squared] difference (24) = 65.28, P less than 0.05]. Several findings of this study point to the need to design programs to motivate adolescent females to adopt healthy weight-control practices and to target young peoples' social networks to promote health behaviors, especially with regard to smoking. (Contains 2 figures and 2 tables.)